Joomla provides a sophisticated database abstraction layer to simplify the usage for third party developers. New versions of the Joomla Platform API provide additional functionality which extends the database layer further, and includes features such as connectors to a greater variety of database servers and the query chaining to improve readability of connection code and simplify SQL coding.

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Usage

Joomla can use different kinds of SQL database systems and run in a variety of environments with different table-prefixes. In addition to these functions, the class automatically creates the database connection. Besides instantiating the object you need just two lines of code to get a result from the database in a variety of formats. Using the Joomla database layer ensures a maximum of compatibility and flexibility for your extension.

The Connection

Joomla supports a generic interface for connecting to the database, hiding the intricacies of a specific database server from the Framework developer, thus making Joomla code easier to port from one platform to another.

The getDbo method will return a reference to a high-level abstract class called JDatabaseDriver; the Joomla Framework hides the specifics of the database server, which is loaded dynamically by getDbo via Joomla's configuration file.

users should note that the JModel class has been renamed JModelLegacy and will eventually be deprecated in favour of the class JModelBase. Therefore, any Joomla 3.0 core or extension code should use JFactory::getDbo() to obtain a database connection.

The Query

Joomla's database querying has changed since the new Joomla Framework was introduced "query chaining" is now the recommended method for building database queries (although string queries are still supported).

Query chaining refers to a method of connecting a number of methods, one after the other, with each method returning an object that can support the next method, improving readability and simplifying code.

To obtain a new instance of the JDatabaseQuery class we use the JDatabaseDriver getQuery method:

$db= JFactory::getDbo();$query=$db->getQuery(true);

The JDatabaseDriver::getQuery takes an optional argument, $new, which can be true or false (the default being false).

To query our data source we can call a number of JDatabaseQuery methods; these methods encapsulate the data source's query language (in most cases SQL), hiding query-specific syntax from the developer and increasing the portability of the developer's source code.

Some of the more frequently used methods include; select, from, join, where and order. There are also methods such as insert, update and delete for modifying records in the data store. By chaining these and other method calls, you can create almost any query against your data store without compromising portability of your code.

Selecting Records from a Single Table

Below is an example of creating a database query using the JDatabaseQuery class. Using the select, from, where and order methods, we can create queries which are flexible, easily readable and portable:

// Get a db connection.$db= JFactory::getDbo();// Create a new query object.$query=$db->getQuery(true);// Select all records from the user profile table where key begins with "custom.".// Order it by the ordering field.$query->select(array('user_id','profile_key','profile_value','ordering'));$query->from('#__user_profiles');$query->where('profile_key LIKE \'custom.%\'');$query->order('ordering ASC');// Reset the query using our newly populated query object.$db->setQuery($query);// Load the results as a list of stdClass objects.$results=$db->loadObjectList();

Chaining can become useful when queries become longer and more complex.

Selecting Records from Multiple Tables

Using the JDatabaseQuery's join methods, we can select records from multiple related tables. The generic "join" method takes two arguments; the join "type" (inner, outer, left, right) and the join condition. In the following example you will notice that we can use all of the keywords we would normally use if we were writing a native SQL query, including the AS keyword for aliasing tables and the ON keyword for creating relationships between tables. Also note that the table alias is used in all methods which reference table columns (I.e. select, where, order).

// Get a db connection.$db= JFactory::getDbo();// Create a new query object.$query=$db->getQuery(true);// Select all articles for users who have a username which starts with 'a'.// Order it by the created date.$query->select(array('a.*','b.username','b.name'))->from('#__content AS a')->join('INNER','#__users AS b ON (a.created_by = b.id)')->where('b.username LIKE \'a%\'')->order('a.created DESC');// Reset the query using our newly populated query object.$db->setQuery($query);// Load the results as a list of stdClass objects.$results=$db->loadObjectList();

The join method above enables us to query both the content and user tables, retrieving articles with their author details. There are also convenience methods for inner, left, right and outer joins.

We can use multiple joins to query across more than two tables:

$query->select(array('a.*','b.username','b.name','c.*','d.*'))->from('#__content AS a')->join('INNER','#__users AS b ON (a.created_by = b.id)')->join('LEFT','#__user_profiles AS c ON (b.id = c.user_id)')->join('RIGHT','#__categories AS d ON (a.catid = d.id)')->where('b.username LIKE \'a%\'')->order('a.created DESC');

Notice how chaining makes the source code much more readable for these longer queries.

Inserting a Record

The JDatabaseQuery class provides a number of methods for building insert queries, the most common being insert, columns and values.

// Get a db connection.$db= JFactory::getDbo();// Create a new query object.$query=$db->getQuery(true);// Insert columns.$columns=array('user_id','profile_key','profile_value','ordering');// Insert values.$values=array(1001,$db->quote('custom.message'),$db->quote('Inserting a record using insert()'),1);// Prepare the insert query.$query->insert($db->quoteName('#__user_profiles'))->columns($db->quoteName($columns))->values(implode(',',$values));// Reset the query using our newly populated query object.$db->setQuery($query);

Since Joomla 3.0 there has been a change in how the Joomla database class executes a query.

When executing a query in the Joomla Framework 11.4 we use the JDatabase's query method:

Since the phase out of PHP 4.x support, PHP's exception model has been widely implemented in the Joomla Framework. Therefore, we can wrap our database execution in a try...catch block to catch any unexpected database errors that may arise. Any database-specific errors will be thrown using an instance of the JDatabaseException class although it is best practice to catch the generic Exception object from which JDatabaseException inherits.

Updating a Record

This is just a simple example to update the params column in the menu table.